a) Field of the invention
The present invention relates to a zoom lens system for lens shutter cameras, and more specifically to a zoom lens system which has high vari-focal ratio, and is nevertheless compact in external design, high in optical performance and manufacturable at a low cost.
b) Description of the prior art
The zoom lens system according to the present invention has a high vari-focal ratio within a range of focal length from 38 mm to 105 mm. As conventional examples of the zoom lens systems having vari-focal ratios on the similar order, there are known the zoom lens systems listed below:
(1) The zoom lens systems each consisting of a first positive lens unit and a second negative lens unit as exemplified by the lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-50118.
(2) The zoom lens systems each consisting of a first positive lens unit, a second positive lens unit and a third negative lens unit as exemplified by the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 63-153511, Kokai Publication No. Hei 2-16515 and Kokai Publication No. Hei 2-73211.
(3) The zoom lens systems each consisting of a first negative lens unit, a second positive lens unit and a third negative lens unit as exemplified by the lens systems disclosed by Japanese Patent Kokai Publication No. Sho 64-72114, Kokai Publication No. Hei 2-37317 and Kokai Publication No. Hei 2-63007.
(4) The zoom lens systems each consisting of a first positive lens unit, a second negative lens unit, a third positive lens unit and a fourth negative lens unit as exemplified by the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 63-43115 and Kokai Publication No. Hei 1-252915.
Out of the conventional zoom lens systems mentioned above, the type (1) has a simple composition but, when its vari-focal ratio is enhanced, both the lens units are moved for long distances, thereby enlarging external design of the zoom lens system.
Further, the type (4) comprises a large number of lens units. Accordingly, this type of zoom lens system requires a complicated mechanism for moving the lens units and arrangement of the lens units with very little eccentricity, thereby having a disadvantage from the viewpoint of manufacturing.
Furthermore, the type (2) or (3) which consists of the three lens units can be designed as a compact zoom lens system. However, the type (3) which comprises the negative lens unit on the most object side is not suited for a compact design since the moving distance for the lens units are largely different from one another and the mechanism for moving the lens units can hardly be simplified in this zoom lens system. In contrast, the type (2) which comprises the positive lens unit on the most object side allows the first lens unit and the third lens unit to be moved integrally, and the moving mechanism to be simplified since the moving distance for the first lens unit is little different from that for the third lens unit.
Out of the conventional zoom lens systems described above, the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 63-153511, Kokai Publication No. Hei 1-230013 and Kokai Publication No. Hei 2-73211 have long total lengths and are not compact in external designs.
In addition, the zoom lens system proposed by Japanese Patent Kokai Publication No. Hei 2-16515 has a short total length, but aberrations therein are not corrected sufficiently for practical use of this zoom lens system.